The alumina content is up to 95% by weight in conventional alumina continuous filament woven fabrics, and this is because a fiber composed of high-purity alumina having an alumina content of at least 95% by weight is very brittle, and thus it is difficult to prepare a continuous filament from this alumina fiber, and even if a high-purity alumina continuous filament is obtained, it is difficult to form a woven fabric from this continuous filament by using a loom.
An alumina continuous filament heretofore used for woven fabrics comprises a silica (SiO.sub.2) component and the like, and generally has an alumina content of 75 to 80% by weight, the alumina content is 95% by weight at highest. This alumina continuous filament generally has a tensile strength of 150 to 200 kg/mm.sup.2 and is less brittle than the high-purity alumina continuous filament having an alumina content of at least 95% by weight, and thus this continuous filament can be woven relatively easily. Nevertheless, the modulus of elasticity of this alumina continuous filament is generally 15 to 20 t/mm.sup.2, and is much lower than 30 to 45 t/mm.sup.2 of the high-purity alumina fiber, and this alumina continuous filament has a disadvantage of a low heat resistance. Accordingly, a woven fabric formed by using an alumina continuous filament having an alumina content lower than 95% by weight has a disadvantage in that only a composite material having a low modulus of elasticity and a low heat resistance is obtained, even if it is used as a reinforcing fibrous substrate of a fiber-reinforced plastic, metal or ceramic material. Thus, a woven fabric formed by using a high-purity alumina continuous filament is highly suitable as a reinforcing fibrous substrate.
It is expected that, if a hybrid woven fabric is obtained by combining a high-purity alumina continuous filament having a very high tensile modulus of elasticity with a high-strength continuous filament having a tensile strength of 200 kg/mm.sup.2 or higher, this woven fabric will have superior physical properties such a high strength, and a high modulus of elasticity will be obtained, and this woven fabric will be very satisfactory as a reinforcing fibrous substrate for a composite material.
As the process for preparing an alumina continuous filament composed mainly of alumina (an alumina content of at least 80% by weight), there can be mentioned, for example, a process in which a spinning solution comprising an aluminum salt, a water-soluble polysiloxane and a water-soluble polymer is spun and then calcined, a process in which a solution comprising an alumina sol, a silica sol and boric acid is spun as the spinning solution, and a process in which a mixture of an inorganic polymer having a polyaluminoxane (--Al--O--) group and a silicic acid ester is spun as the spinning solution. In each of these processes, however, a silica component (SiO.sub.2) is incorporated in an amount of at least 5% by weight, to lessen the brittleness of the alumina fiber.
As the process for preparing a high-purity alumina fiber having an alumina content of at least 95% by weight, there is known a process in which a slurry comprising a-alumina particles having a particle size smaller than 0.5 tm, a small amount of MgCl.sub.2 ;6H.sub.2 O and a basic aluminum salt is dry-spun and the spun filament is calcined at a temperature higher than 1000.degree. C., as proposed in Japanese Unexamined Patent Publication No. 49-35627. Also in the fiber obtained according to this process, the brittleness is generally lessened by a coating of amorphous SiO.sub.2.
An alumina continuous filament woven fabric is prepared by weaving an alumina continuous filament prepared as mentioned above. At the warping step or picking step in the weaving process, the alumina continuous filament is brought into contact with a guide or the like, a filament breaking or a formation of fluffs occurs, and a good weave becomes difficult. As the weaving process in which a filament breaking or formation of fluffs is prevented, and the weaving property is improved, there is generally adopted a process in which an alumina continuous filament having a lesser brittleness due to an incorporation of a silica component or the like in an amount of at least 5% by weight, usually 15 to 25% by weight, is used. Furthermore, Japanese Unexamined Patent Publication No. 62-170522 proposes a process in which a precursor fiber obtained by spinning a spinning solution containing a precursor is woven, and the woven fabric is heat-treated to obtain an alumina continuous filament woven fabric. Nevertheless, also in the fiber of the woven fabric obtained according to this process, the alumina content is restricted to less than 95% by weight because of problems arising in the preparation of the precursor, and since the calcination is carried out after the weaving, the mesh of the obtained woven fabric is loose and shearing readily occurs.
As pointed out above, since a high-purity alumina continuous filament is very brittle, it is very difficult to form a woven fabric from this continuous filament by using a loom. Accordingly, only a woven fabric having an alumina content lower than 95% by weight has been provided as the woven fabric comprising an alumina continuous filament, and a satisfactory woven fabric comprising a high-purity alumina continuous filament having an alumina content of at least 95% by weight, or a weaving process for forming such a woven fabric stably at a high efficiency, have not been proposed.